Liquid crystal lenses and other optical devices are known in the art. One known geometry is a planar construction in which liquid crystal is held in a cell between glass or plastic plates. An electrically variable GRadient INdex (so called GRIN) lens can be provided by having a spatial variation of the index of refraction of the liquid crystal material across (within) the aperture of the device. Good optical power of a lens can be achieved without substantive thickness.
A variety of liquid crystal lens designs have been proposed affecting, with spatial variation, the way in which the liquid crystal is oriented in response to an electric field. U.S. Pat. No. 7,667,818 to Galstian et al. and U.S. Pat. No. 6,864,951 to Ren et al., which are incorporated herein by reference, disclose different techniques for spatially influencing how a uniform electric field orients liquid crystal (molecules) in a layer. Commonly-assigned PCT patent application publication WO/2010/006420 published Jan. 21, 2010, which is incorporated herein by reference, teaches a spatial modulation of an alignment layer on a liquid crystal cell wall to form a zero voltage lens that can be reduced or erased by applying a spatially uniform electric field.
Most designs, however, spatially modulate an electric field acting on the liquid crystal layer to create a resulting GRIN lens. In this area, a few approaches have been taken. Using relatively large voltages, it has been shown that a ring electrode placed at a distance above a liquid crystal cell under which a planar electrode is arranged, can provide a GRIN lens. In an article published by A. F. Naumov et al., titled “Liquid-Crystal Adaptive Lenses with Modal Control” OPTICS LETTERS, Vol. 23, No. 13, Jul. 1, 1998, edge (ring) electrodes are arranged with an electrically resistive coating of titanium oxide placed on a glass substrate with the liquid crystal alignment layer placed on the electrode coating, essentially as shown in FIG. 1A. The GRIN lens taught by Naumov et al. has a liquid crystal cell with a distributed reactive electrical impedance. The conductance and the capacitance of the liquid crystal between the electrodes play an important role in the distributed reactive electrical impedance. The resistance of the titanium oxide coating is between 2 and 10 MΩ/□. Lens focal length is essentially controlled by voltage at higher optical powers, while at lower optical power, it is reported that both frequency and voltage can be used to control optical power. That is lens control is very complex.
Optical and electrical performance of the Naumov et al. lens design are good, however, a significant drawback is that the resistive coating is difficult to manufacture to have reproducible (part-to-part) and uniform properties. FIG. 1B illustrates resistive characteristics of the resistive coating. The resistance bulk material property becomes relevant as a sheet resistance of the resistive coating. The sheet resistance undergoes high variability at coating thicknesses which enable the operation of such liquid crystal lenses or optical devices. Optically suitable coating materials have been found to exhibit the required sheet resistance in a percolation zone where minute differences in resistive coating thickness in manufacturing terms, result in very large sheet resistance variability.
Research directed by Susumu Sato has led to a design using a ring electrode on one side of the liquid crystal with a planar electrode on the other side, in addition to a planar electrode on top of the ring electrode. This geometry was also shown to benefit from the use of resistive coating placed between the liquid crystal and the ring shaped electrode, see for example, “Reducing Driving Voltages for Liquid Crystal Lens Using Weakly Conductive Thin Film” by Mao Ye, Bin Wang, Maki Yamaguchi, and Susumu Sato, published in Japanese Journal of Applied Physics, Vol. 47, No. 6, 2008, pp. 4597-4599.
In PCT patent application publication WO2007/098602 published Sep. 7, 2007 to Galstian et al., which is incorporated herein by reference, a liquid crystal lens uses uniform planar electrodes with an electric field modulation layer that is optically hidden, while spatially modulating the electric field due to a non-uniform dielectric constant.
In commonly-assigned PCT patent application publication, WO2009/153764 published Dec. 23, 2009, which is incorporated herein by reference, a ring electrode is placed on one side of a glass substrate with an alignment layer on an opposite side of the glass substrate. A weakly conductive layer is provided on or near the ring electrode to create a charge spatial distribution across (over) the aperture. The optical power of the lens can be controlled from zero to maximum optical power by varying (using) the frequency of the control signal. The present assignee, LensVector Inc., has demonstrated lenses of 15 diopters, 2 mm aperture and a total thickness of about 0.5 mm, with an operating voltage of about 28 V.
In the case of a ring electrode that uses a frequency dependent material, a highly resistive material, or a weakly conductive material (hereinafter called a weakly conductive material) placed near the aperture, the electrical (or sheet) resistance of the material plays an important role in defining the electrode and lensing properties. Controlling the resistance of a thin layer of material on a wafer is a challenge, while the resistance or conductive properties are very important to frequency control of the electrode.